Separations of target compounds of microspecies (MW&lt;1000 amu), macromolecules (MW&gt;1000 amu) and/or macromolecular structures (including but not limited to micelles, reverse micelles, metal complexes) is generally accomplished with reverse osmosis, microfiltration, ultrafiltration, nanofiltration and combinations thereof. These methods suffer from high pressure drop and low flow rate.
Use of supercritical fluids as a solvent generally involves a depressurization so that the solvent quickly evaporates leaving behind the target compound. This method suffers from the inefficiency introduced by the depressurization.
U.S. Pat. No. 5,430,224 to Schucker discusses a supercritical perstraction process. Supercritical carbon dioxide is reported to increase toluene permeation through a polyesterimide membrane compared to n-heptane.
The paper Supercritical CO.sub.2 Extraction Coupled With Nanofiltration Separation Applications to Natural Products, S J Sarrade, G M Rios, M Carles, Separation and Purification Technology 14 (1998) 19-25, states that the idea of coupling a supercritical CO.sub.2 extration stage with nanofiltration separation to extract and purify low molecular weight compounds up to 1500 g/mol had been proposed as early as 1994. In this paper, Sarrade et al. discuss fractionation of fish oil to obtain essential fatty acids useful in treating heart disease, and purification of .beta.-carotine from either carrot oil or carrot seeds.
The International Publication WO 96/18445, Jun. 20, 1996 by Sarrade et al. contains discussion of additional separations using supercritical fluids, for example triglycerides from butter, but again is limited to target compounds with molecular weights ranging from 50 to 1,000 Daltons.
Hence, there is a need for a method of separating a target compound having a macromolecular structure from a solvent with greater efficiency.